In automotive vehicles such as trucks, tractors and cars, it is common to provide a drive linkage between the vehicle engine and the drive wheels. This generally includes a line of rotating components from the rotating output of the engine to the rotating input to the drive wheels. In order to vary the ratio of speeds between the engine output and drive wheel input, a transmission device is typically used where the same can be shifted to give the operating speed or power ratios required. There further generally is a means for interrupting power transmission between the engine output and the drive wheels, and this for example takes the form of a clutch, a torque converter, or a fluid coupling.
A transmission device in its simplest form has two adjacent parallel shafts, one forming the input shaft to which power is applied and the other forming the output shaft which is ultimately connected, typically through a differential mechanism or the like, to the drive wheels. There typically would be at least two pairs of mated gears mounted respectively on the input and output shafts, where the respective pairs of gears are continuously engaged with one another. Further, one gear of each pair is keyed nonrotatably to its respective shaft while the other gear is freely rotatable on its respective shaft. Thus, although the gears are continually meshed, with nothing more, rotation of the input power shaft does nothing to cause rotation of the output shaft.
To selectively key the rotatably mounted gear then to its respective shaft, a gear clutch or synchronizer located adjacent the gear is used. In one common form of a gear clutch, a coupling member is nonrotatably keyed to, but is movable axially on the shaft, into or out of engagement with the adjacent gear. A shifting linkage is manually activated to provide this axial shifting. Cooperating teeth means on the coupling member and gear are engaged then in the drive position to key the gear via the coupling member to the shaft. With this concept, the teeth means must be synchronized before shifting can take place, and normally a main clutch is needed to accomplish this and to interrupt torque transmission through the gear set. However, inasmuch as the coupling member is moved under a positive force through direct linkage, an operator can attempt to or accidentally override or force shift the transmission to bind and/or damage teeth means if they are not in proper synchronization. Consequently, all components must be overdesigned with considerable size and bulk in order to withstand these possible idiosyncrasies of operation, which in turn adds tremendous weight and expense to the cost of the vehicle.
In a more elaborate form of a gear clutch, various means synchronize the teeth means automatically. This reduces the possibility of an operator damaging the teeth means by forcing them into engagement when they are not completely synchronized, but it adds complexity to the design. Thus, commonly a second sleeve type member is positioned between the interfaces of the coupling member and gear which has a friction type action on the freely rotating component to induce a quicker synchronization; while further which has a blocking action on the components to prevent the engagement of the teeth means until synchronization is achieved. This complicates the structure and further adds cost and an additional possible source of failure.
There further is the problem in many of these gear clutch mechanisms that all torque must be removed from the drive train before the gear clutch can be disengaged. This is in part true because the engaged coupling teeth, when under a torquing condition, generate frictional forces holding them together. Under such circumstances, it is necessary to interrupt power transmission in order to disengage one gear clutch and shift to a different drive ratio through another gear clutch.
Patents which illustrate these various forms of gear clutches are as follows: Carnagua et al. U.S. Pat. No. 2,459,360; Dodge U.S. Pat. No. 2,735,528; Pawlina et al. U.S. Pat. No. 3,333,661; Vollmer U.S. Pat. No. 3,611,832; McNamara et al. U.S. Pat. No. 3,648,546; Richards U.S. Pat. No. 3,910,131; Richards U.S. Pat. No. 3,921,469; Richards U.S. Pat. No. 3,924,484; and Richards U.S. Pat. No. 3,983,979.